The present invention relates to a process which makes it possible to control the viscosity or the average molecular mass of a polyisobutene produced continuously in a reactor in liquid phase.
It is known to polymerize isobutene continuously in a reactor comprising a boiling liquid reaction phase containing the monomer and the polymer being formed, above which there is a gas phase comprising, in particular, the monomer which is in equilibrium with the liquid phase. The continuous polymerization is brought about in particular by continuous feeds into the reactor of the monomer and of a catalyst and by continuous withdrawal from the reactor of the liquid phase, which is, generally, subjected subsequently to one or more purification steps which are intended to isolate the polyisobutene produced.
The monomer often consists of isobutene originating from a mixture of butenes and/or butanes.
In general, the polymerization reaction is conducted continuously with the aid of a catalyst of cationic type and, if appropriate, of a cocatalyst.
In a continuous polymerization, the monomer, i.e. isobutene, is generally supplied by means of an essentially C4 hydrocarbon cut; that is to say, a mixture comprising isobutene, other C4 olefins and/or C3 to C7 alkanes, especially C4 alkanes. The quality of the monomer supply may vary over time, such that it adversely affects the polymerization conditions and, consequently, the quality of the polymer obtained.
The applications of polyisobutenes are often linked to their rheological properties. One of the essential characteristics of polyisobutene is its viscosity or its average molecular mass.
In a continuous polyisobutene production process, the average residence time of the polymer in the polymerization reactor can be relatively long. Moreover, the reaction mixture withdrawn continuously from the polymerization reactor enters one or more polymer purification steps. The final polymer is therefore isolated and purified after an additional time which may generally be a number of hours, for example from 3 to 12 hours, such that any analysis of the polymer at the end of this last step is carried out very late. Consequently, the time elapsed between a deviation measurable from the analysis of the viscosity or of the average molecular mass of the polyisobutene, and the correction of the said-deviation in the polymerization reactor, is relatively great. This type of deviation therefore gives rise to the production of product which is outside the specifications of viscosity or average molecular mass, generally in a not inconsiderable amount.
Methods have been investigated in the past to partially solve the above mentioned problem.
In the process of the French Patent Application 2 625 506, a method is disclosed to determine one or more polymer properties using a correlative relation with absorption measurements carried out on the polymer with an infrared spectrophotometer. A process control using this method is also disclosed but it does not address the problem solved by the present invention.
The U.S. Pat. No. 4,620,049 describes a method adapted for controlling the molecular weight of a product output from a polybutene reactor. The method in particular comprises determining a formula correlating molecular weight simultaneously with temperature of the reactor and concentration of isobutene in the reactor. The desired product molecular weight is then obtained by altering, through the use of the formula, the temperature of the reactor and/or the concentration of isobutene in the reactor. However the principle of this method does not comprise maintaining constant a corrected value of the isobutene partial pressure in the reactor gas phase, in particular independently of the polymerization temperature. Moreover, involving the temperature of the reactor in the formula of this method implies that the temperature may vary even slightly and therefore affects the quality of polyisobutene produced, such as the unsaturated termination content of the polymer.
The technical problem to be solved is to find a process control which makes it possible to correct the fluctuations in viscosity or average molecular mass of the polyisobutene and thus to intervene more rapidly in the conditions of the polymerization in the reactor in order to limit the quantity of polyisobutene which is produced outside the specifications.
This problem was partially solved by the process described in French Patent Application Filing No. 9903267, which makes it possible to maintain a property P at a constant value, the property P being selected from the viscosity or the average molecular mass, firstly by determining a target value V for the isobutene partial pressure PiC4 in the gas phase of the reactor, which corresponds to the desired value of the property P, and secondly by maintaining the said partial pressure at a constant value around the target value V by acting on the flow rate Qc of the catalyst introduced into the reactor and/or on the flow rate Qh of the C4 hydrocarbon feed mixture.
The invention described in French Patent Application Filing No. 9903267 nevertheless presents possibilities for improvement. Indeed, despite the maintenance of the isobutene partial pressure PiC4 at a constant value it has been observed that the property P sometimes has a tendency to deviate. Consequently, it is often necessary to readjust the said partial pressure, which often results in the production of product which is outside the specification.
The process control based on maintaining the partial pressure PiC4 at a constant value was employed in the process of the patent application owing in particular to the difficulty of measuring the concentration of isobutene in the reactive liquid phase of the reactor. The partial pressure PiC4 can be considered as a weighted image of the concentration of isobutene in the reactive phase, in accordance with the laws of liquid/vapour equilibrium.
It has been found that various parameters acting on the reaction mixture are able to modify this liquid/vapour equilibrium and influence the partial pressure PiC4 without directly affecting the property P of the polyisobutene produced.
The task was therefore undertaken of improving the process by researching which were the determining parameters which influenced the liquid/vapour equilibrium, in order to correct the partial pressure PiC4 and so to avoid the undesirable effects referred to above. The partial pressure PiC4 value corrected in this way can become independent of the liquid/vapour equilibrium and can thus be used to control the property P with greater reliability.
The subject of the present invention therefore lies in a process which involves using new parameters in an improved control, and in particular lies in a modelling of the partial pressure PiC4 which thereby makes it possible in particular to improve the process described in French Patent Application Filing No. 9903267.
The present invention relates to a process for maintaining a property P of a polyisobutene at a constant desired value in the course of an isobutene polymerization conducted continuously in a reactor comprising a boiling liquid reaction phase which contains the monomer and the polymer being formed and is in equilibrium with a gas phase on top of the said liquid phase, the polymerization being conducted by continuous introduction into the reactor of a catalyst and of a C4 hydrocarbon feed mixture comprising the monomer, and by continuous withdrawal from the reactor of the liquid reaction phase, which is subsequently subjected continuously to at least one purification step which is intended to isolate the polyisobutene produced, this process being characterized in that the property P is selected from the viscosity and the average molecular mass of the polyisobutene produced and in that, by virtue of an empirical relationship established beforehand between the property P of the polyisobutene produced and the partial pressure PiC4 of the isobutene in the gas phase of the reactor, a target value V is determined for PiC4, corresponding to the desired value of the property P, and in that, during the polymerization, the partial pressure PiC4 in the gas phase of the reactor, and at least one of the parameters selected from the polymerization temperature and the concentration of at least one of the constituents of the C4 hydrocarbon feed mixture, are measured, a corrected value of the isobutene partial pressure, (PiC4)c, is calculated from the measured value of PiC4 and from that of at least one of the said parameters, and the said corrected value (PiC4)c is held constant at around the said target value V by acting on the flow rate Qc of the catalyst introduced into the reactor and/or on the flow rate Qh of the C4 hydrocarbon feed mixture introduced into the reactor.